Yes, they are maximum values. Denser sampling will usually help because it provides a better oversampling of the diffraction pattern, which in turn allows more accurate restoration by deconvolution sharpening.
Yes, that's how it happens to work out for high contrast detail. But do keep in mind that lower contrast subject matter will be mostly lost already at wider apertures than these limiting ones. Stopping down will kill low contrast micro-detail long before it kills all micro-detail (of any level of contrast). For the D800 it starts at f/5.6 and for the D3 it starts at f/9 .
Here we're getting into even higher degrees of speculation. How low is low-contrast? Also, at some predefined level of lowness, the strength of the camera's AA filter will have a more significant effect on the results. Many people were surprised to see how little resolution difference there seemed to be between the D800 and the D800E in the sample images on display. At a particular low level of contrast, I imagine the D800E would produce a more obvious improvement over the D800, than it does at high contrast.
What I propose is a test chart with the usual B&W lines spaced at progressively smaller intervals, then another columm on the same sheet containing the same size and spacing of lines, but dark grey and white lines, then another column of medium grey and white lines, then pale gray and white lines, then very pale grey and white lines.
Such a chart would enable us to quantify more precisely the different capacities of different cameras, with different strengths of AA filters and different pixel densities, to handle predefined levels of low-contrast micro-detail at specific F stops.
However, I see another problem here. It is understood that one would try to use the same lens on both cameras when doing such a test, but the choice of lens may significantly affect the results, and I'm not referring just to the quality of the lens, in terms of good or bad, cheap or expensive, but rather the lens MTF characteristics and its design.
For example, certain high-contrast lenses, such as some of the Carl Zeiss lenses, are very appealing (and very expensive of course) because they have a high MTF response at relatively low frequencies, say up to 40 lp/mm. However at higher frequencies up to the Nyquist limit of the D800, say 100 cy/mm, the MTF response may be significantly lower in the Zeiss lens than another design of lens, whether the other lens is cheaper or not.
Therefore, when comparing the handling of low-contrast micro-detail with the D800 and D3, for example, the high-contrast lens would favour the D3 and the low-contrast lens would favour the D800. In other words, the relatively high MTF response of a low contrast lens at 70 to 100 lp/mm would be irrelevant with regard to the D3 because the D3 cannot resolve any more than 60 lp/mm, whatever the circumstances. But that additional contrast at high resolution could have a visible impact on the D800 results.